This invention relates to an electrophotographic printing machine, and more particularly to a method and apparatus for removing toner dispersant from an image formed from a liquid developer.
A typical electrostatographic printing machine employs an imaging member that is exposed to an image to be printed. Exposure of the imaging member records an electrostatic latent image on it corresponding to the informational areas contained within the image to be printed. The latent image is developed by bringing a developer material into contact therewith. The developed image recorded on the photoconductive member is transferred to a support substrate such as paper, either directly or via an intermediate transport member. The developed image on the support substrate is generally subjected to heat and/or pressure to permanently fuse it thereto.
Two types of developer materials are typically employed in electrostatographic printing machines. One type of developer material is known as dry developer material and comprises toner particles or carrier granules having toner particles adhering triboelectrically thereto. Another type of developer material is a liquid material comprising a liquid carrier or dispersant having toner particles dispersed therein.
Liquid developer typically contains about 2 percent by weight of fine solid particulate toner material dispersed in the liquid carrier. The liquid carrier is typically a hydrocarbon. In the developing process, the developed image on the photoreceptor contains about 12 weight percent of particulate toner in liquid hydrocarbon carrier. To improve the quality of transfer of the developed image to a receiving member or copy sheet, the image should be conditioned. Conditioning an image includes increasing the percent solids in liquid by removing liquid carrier from the image while preventing toner particles from departing the image; and electrostatically compressing or compacting the toner particles of the image to physically stabilize the image and to produce a clear, high resolution image. Depending on the particular liquid carrier structural composition and its respective properties during the image formation process, e.g. vapor pressure rate, evaporation rate and volatility, the percentage of solids in the liquid should be increased to in the range of 25 to 75 percent. Removing the liquid dispersant minimizes image show-through on a support substrate and prevents problems associated with the image later emerging from the support material. Additionally, increasing the solids content of an image before transferring the image to an intermediate transport member greatly improves the ability of the toner particles to form a high resolution image on the transport member and thus on the support material. However, difficulty lies in separating and removing the liquid dispersant from the liquid developer, without disturbing the toner image. It is a critical step, nonetheless, not only to ensure that a high quality image is ultimately produced on a final substrate, but also to prevent toner particles from entering the liquid dispersant removal device, so that the device may remain clean and free from thereafter contaminating a subsequent image with embedded toner particles. Image offset is a serious instance of this problem where a toner impression of the partial or entire image embeds into the removal device, which may be in the form of a roller, causing it to reappear in a subsequent image, offset from the actual image, and causing blurred or multiple images on the final support substrate.
Increase in percent solids may be achieved by removing excess liquid carrier with a porous blotter in the form of a roller or belt (hereinafter collectively referred to as "roller"), typically positioned with respect to the photoconductive member retaining the latent image. When the developed image is transferred to an intermediate belt before final transfer to a final copy sheet, the developed image on the intermediate belt should again be blotted to further increase the percentage of toner solids, so that the amount of liquid on the final substrate is minimized, and a well defined, high quality image is produced.
A vacuum assisted blotter roller is effectively used to condition an image formed from a liquid developer. A vacuum absorption system is used to draw off liquid toner dispersant such as Isopar from an absorbent material which in turn is used to remove the dispersant from the toner image on an electrostatographic imaging member or intermediate transport member. Several advantages exist by eliminating excess dispersant by vacuuming the liquid through the roller, over other methods of liquid reduction found in the prior art. For example, less dispersant evaporates into the atmosphere, thus reducing pollution and potential health risks to individuals working near the machine. As the dispersant has a potential for being reused, it also makes the device more cost effective. In addition, the formed image is found to be more clearly defined, as the potential for the removed liquid from returning back to the image bearing surface from the roller and disturbing the image is eliminated. A vacuum assisted blotter roller is disclosed in U.S. Pat. No. 5,332,642, having a common assignee as the present application.
Preferably, the roller is electroconductive and a bias is applied to the roller having a potential of the same sign polarity as the toner in the liquid developer, so that the toner is repelled from the roller. By applying a bias potential to the roller, toner particles are prevented from entering the roller, and rather, remain in tact with the image. Furthermore, the toner image is compacted by the bias and/or pressure contact of the roller. The bias may be applied to the rigid, porous supportive core, or may be applied to both the rigid core and the absorbent material formed around the core. A bias applied to a conductive absorbent material, or to individual layers of a multi-layered absorbent material, allow the roller's electrical field to approach more closely to the toner image and thus exert a stronger repelling action than if the biased roller were separated from the toner image by an insulating absorbent material. Polymers such as various polyurethanes, olefins, tetrafluoroethylene, and various elastomers, may be processed into open cell absorbent polymeric foam material appropriate for use in blotter roller applications, using the teachings, for example, in U.S. Pat. Nos. 3,696,180; 3,729,536; 3,860,680; 3,968,292; 4,157,424, and other methods known in the art. Conductive fillers, organic and inorganic, ionic or electronic are useful and may be added to regulate the poromeric material conductivity. The porous supportive core typically is made from a sintered metal, plastic or ceramic, and is electroconductive, either by itself, or in combination with another conductive material.
Various techniques have been devised for removing excess liquid carrier from an imaging member which involve either a vacuum removal system or an electrical bias applied to a portion of the liquid dispersant removal device. The following references may be relevant to various aspects of the present invention.
U.S. Pat. No. 4,286,039 discloses an image forming apparatus comprising a deformable polyurethane roller, which may be a squeegee roller or blotting roller which is biased by a potential having a sign the same as the sign of tire charged toner particles in a liquid developer. The bias on the polyurethane roller is such that it prevents streaking, smearing, tailing or distortion of the developed electrostatic image and removes much of the liquid carrier of the liquid developer from the surface of the photoconductor.
U.S. Pat. No. 5,028,964 discloses an apparatus for image transfer which comprises an intermediate transfer member and a squeegee for removing excess liquid from the toner image prior to transferring an image. The intermediate transfer member is operative for receiving the toner image therefrom and for transferring the toner image to a receiving substrate. Transfer of the image to the intermediate transfer member is aided by providing electrification of the intermediate transfer member to a voltage having the same bias as that of the charged particles. The roller is charged to a potential having the same polarity as the charge of the toner particles of the liquid developer.
U.S. Pat. No. 4,878,090 discloses a development apparatus comprising a vacuum source which draws air around a shroud to remove excess liquid carrier from the development zone.
U.S. Pat. No. 5,023,665 discloses an excess liquid carrier removal apparatus for an electrophotographic machine. The apparatus is comprised of an electrically biased electrode having a slit therein coupled to a vacuum pump. The vacuum pump removes, through the slit in the electrode, liquid carrier from the space between the electrode and the photoconductive member. The electrical bias generates an electrical field so that the toner particle image remains undisturbed as the vacuum withdraws air and liquid carrier from the gap.
Copending file wrapper continuation application for U.S. patent Ser. No. 08/082,141, filed Apr. 12, 1993, and allowed on May 4, 1994, of parent application, Ser. No. 07/779,559, filed Oct. 18, 1991, now abandoned, and both having a common assignee as the present application, discloses a belt used for absorbing liquid toner dispersant from a dispersant laden image on a electrostatographic imaging member or intermediate transfer member. The angle of contact of the absorption belt is adjusted with respect to the image bearing member for maintaining proper cohesiveness of the image and absorption of liquid dispersant. The absorption belt is passed over a roller biased with the same charge as the toner. A pressure roller is in contact with the absorption belt for removal of liquid therefrom.
Copending application for U.S. patent Ser. No. 08/247,737, filed May 23, 1994, and having a common assignee as the present application, discloses a roller comprising an absorption material and a covering, which are adapted to absorb liquid carrier from a liquid developer image. The covering has a smooth surface with a plurality of perforations, to permit liquid carrier to pass through to the absorption material at an increased rate, while maintaining a covering having a smooth surface which is substantially impervious to toner particles yet pervious to liquid carrier so as to inhibit toner particles from departing the image.
U.S. Pat. No. 5,332,642, having a common assignee as the present application, discloses a porous roller for increasing the solids content of an image formed from a liquid developer. The liquid dispersant absorbed through the roller is vacuumed out through a central cavity of the roller. The roller core and/or the absorbent material formed around the core may be biased with the same charge as the toner so that the toner is repelled from the roller while the dispersant is absorbed.
Copending application for U.S. patent Ser. No. 08/107,876, filed Aug. 8, 1993, and having a common assignee as the present application, discloses a roller for removal of excess carrier liquid from a liquid developed image, comprising a rigid porous electroconductive supportive core, a conformable microporous resistive foam material provided around the core, and a pressure controller for providing a positive or negative pressure to the roller.
European Patent Application, publication number 0513820A2, filed May 15, 1992, describes a stabilizing roller comprising a deformable roller member including a soft open cell conductive foam, which is disposed on a biased slip ring formed on the outer surface of a central motor driven metal roller core member. A DC bias is applied to the slip ring, of the same polarity as the toner in the liquid developer, and of an opposite polarity to the counter ions within the isopar carrier fluid for transporting the toner on the surface of the photoreceptor drum.